In recent years, a polymerizable liquid crystal composition has been used in a film having optical anisotropy. A polymerizable liquid crystal composition has optical anisotropy in a liquid crystal state, and alignment of a liquid crystal compound contained is fixed through polymerization. A helical structure is induced by adding an optically active compound to a polymerizable liquid crystal compound or composition, and it is applied to various optical devices depending on the pitch of the helical structure.
Examples of the application utilizing selective reflection of visible light include a design purpose, such as an ornamental material, and a color filter used in a liquid crystal display device. The application thereof also includes a anti-counterfeit technology utilizing such features in that reflected light and transmitted light have peculiar metallic gloss with change in color depending on viewing angle, and the optical characteristics cannot be duplicated with an ordinary duplicator. Furthermore, owing to application of the circularly polarized light separation function, such a luminance improving film is proposed that has a structure containing a quarter-wavelength plate and an optical anisotropy film exhibiting the circularly polarized light separation function, which are laminated on a polarizing plate. In these applications, the circularly polarized light separation function is demanded to be expressed over the whole visible light region (i.e., a region having a wavelength of from 350 to 750 nm), and accordingly, plural layers having different pitches are laminated, or the pitch is changed continuously in the thickness direction of the film (as described in Y. Hisatake, et al., Asia Display/IDW '01 LCT8-2).
Examples of the application as an optical compensation film of a liquid crystal display include optical compensation in an STN (super twisted nematic) liquid crystal display (as described in JP-A-2002-6138). A film having a fixed cholesteric alignment layer that transmits a visible light and reflects an ultraviolet ray having a wavelength range of 350 nm or less is referred to as a negative C-plate. A negative C-plate is used as an optical compensation plate suitable for improving viewing angle characteristics of a display device having such a mode as VA (vertically aligned), TN (twisted nematic), OCB (optically compensated birefringence) and HAN (hybrid aligned nematic). A negative C-plate can be used for viewing angle compensation of a VA mode as a combination with an optical compensation layer exhibiting positive birefringence, such as a positive A-plate (as described in JP-A-10-153802 and JP-A-10-152782).
In all the aforementioned purposes, development of such a photopolymerizable cholesteric liquid crystal composition that has the following characteristics is demanded. As characteristics of the photopolymerizable liquid crystal before curing, the composition has a cholesteric phase at room temperature, shows good alignment property, and has quick curing property upon irradiation of UV light, and as characteristics after curing, the composition has a suitable Δn, has transparency, and is excellent in heat resistance and humidity resistance.
In addition to the aforementioned optical characteristics, it is necessary to satisfy demands relating to polymerizability and physical and chemical characteristics of a polymer, upon optimizing the compound. The characteristics includes the polymerization rate of the compound, the polymerization degree, and the transparency, the mechanical strength, the coating property, the solubility, the crystallinity, the contractility, the water permeability, the water absorptivity, the gas permeability, the melting point, the glass transition point, the clearing point, the heat resistance and the chemical resistance of the polymer.
The inventors have proposed a cholesteric polymerizable liquid crystal composition having a cyclic ether, such as oxiranyl and oxetanyl, as a polymerizable group (as described in JP-A-2005-263778). The compound shows such favorable characteristics in that it can be cured with UV light in open air and shows good adhesion to a saponified triacetylcellulose film. However, the achiral polymerizable liquid crystal compound used as the major component of the composition (as described, for example, in JP-A-2005-60373 and Liquid Crystal, vol. 31, No. 12, pp. 1627-1637 (2004)) requires a long production process with high cost.